What is the purpose of mitosis?  

The purpose of mitosis in organisms are endless. But some examples are for tissue repair, growth, asexual reproduction, etc.

Mitosis is the process of cell division where 2 daughter cells are produced and theyre genetically identical to their parent cell. That means, the new cells are completely same to the older cells, same number of chromosomes, same DNA, same function. Therefore, mitosis is super useful for repairing damages tissues, or for growth, where the process of mitosis prevents us from turning into another organism, as the new cells are same as the old ones, and asexual reproduction, where the new offsprings are completely identical to the parent, such as the reproduction of bacteria.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.